"570 ON THE THEORY OF THE VERTEBRATE SKULL 



the extent to which it is absolutely in contact with any one of these 

 ■bones varying. 



The prefrontal and lacrymal bones are always developed in or 

 ■upon that lateral process of the ethmosphenoidal plate, which gives 

 attachment externally to the palatopterygoid arch ; consequently they 

 lie at the anterolateral ends of the frontal, and have more or less close 

 relations with it, the ethmoid and the palatine bones. 



Finally, the nasal bones (or bone) never enter into the composition 

 'of the walls of the skull, but have the same relation to the anterior 

 and upper expanded edge of the prolonged lamina perpendicularis or 

 body of the ethmoid, as the vomer or vomers have to its lower edge. 



If the conclusions which I have laid before you are correct, the 

 following propositions are true of all the bony skulls of Vertebrata. 



1. Their axis contains at most five distinct bones, which are, from 

 before backwards, the basioccipital, the basisphenoid, the presphenoid, 

 the ethmoid, and the vomer ; but any of these bones, except the 

 basisphenoid, may be represented by cartilage, and they may an- 

 chylose to an indefinite extent ; so that the number distinguishable as 

 separate bones in any skull cannot be predicated. The craniofacial 

 axis invariably presents the same regions, but the histological character 

 •of these regions may vary. 



2. Their roof contains at most, leaving Wormian bones out of 

 consideration, five bones (supraoccipital, parietals and frontals), or 

 seven, if we include the epiotic bones in the roof The number falls 

 below this in particular cases, for the same reason as that given for 

 ■the apparent variations in composition of the axis. 



3. Their inferolateral wall contains at most six pair of bones 

 '(exoccipitals, mastoids, petrosals, alisphenoids, orbitosphenoids, pre- 

 frontals), whose apparent number, however, is affected by the same 

 ■causes. 



4. The axial bones have definite relations^ to the brain and nerves. 

 The basioccipital lies behind the pituitary body, the basisphenoid 

 /beneath it, the presphenoid in front of it. In fact the pituitary body 

 may be regarded as marking the organic centre, as it were, of the 

 skull — its relations to the axial cranial bones being the same, as far 

 .as I am aware, in all Vertebrata. 



The olfactory nerves pass on either side of the ethmoid, which 

 bounds the cranial cavity in front, the greater part of its substance and 

 that of the vomer being outside the cranial cavity. 



5. The lateral bones have definite relations to the brain, nerves, 

 and organs of sense. The exoccipital lies behind the exit of the par 



